1. Field of the Invention
The present invention relates to electronic filters, and, more particularly, to switched active filters. Filters in accordance with the present invention have been found to be especially useful in data acquisition circuits of seismic exploration systems.
2. Description of the Prior Art
Electronic filters are devices which transmit, i.e., "pass", input signals of certain frequencies, while substantially attenuating input signals of other frequencies. For example, a low-pass filter will transmit signals below a certain frequency (the cutoff frequency), while attenuating signals above the cutoff frequency.
Two categories of filters commonly in use today are active and passive. As the name implies, an active filter is one having one or more active elements, the most common example of an active element being an amplifier. In addition to active elements, an active filter also includes resistance-capacitance (R-C) networks, and the values of the product of resistance and capacitance in those networks determine the transfer function of the filter, which is the mathematical relationship between the output and input of the filter.
In certain applications, e.g., seismic exploration, it is desirable to be able to vary the cutoff frequency of a given filter over a range of frequencies. One method whereby the cutoff frequency may be varied is by statically switching a different set of fixed components (e.g., resistors and/or capacitors) into or out of the filter circuit. Since each time constant (R-C product) in the filter must be scaled by the same amount in order to maintain the same transfer function scaled to a different frequency, one switch and one component must be provided for each pole of the filter and each change in frequency. It is often the case that the static switching network described above is implemented with a rotary switch, with the fixed components being connected to the terminals of the switch.
Yet another method whereby the cutoff frequency of a filter may be varied is by using a duty cycle controlled filter. Such duty cycle controlled filters include one or more electronic switches which are closed during the on state of a periodic switching signal and opened during the off state of the switching signal. Under appropriate conditions, the duty cycle of the periodic switching signal (i.e., ratio of "on" time to total period) determines the effective time constant or time constants of the filter.
The opening and closing of the switches in duty cycle controlled filters produce transient voltages, which can cause distortion, if they are allowed to enter an amplifier unimpeded. Thus, while such filters have been available, they have not been used in applications requiring low levels of distortion, e.g., seismic exploration. This shortcoming has been overcome with the present invention.